Turbochargers are generally used to enhance operability of a device. For example, in the context of turbine engines, turbochargers may be used to increase pressurization, or to boost, an intake air stream into a combustion chamber. In this regard, hot exhaust gas from the engine may be routed into a turbocharger turbine housing within which a turbine is mounted. The exhaust gas flow impinges against the turbine to cause it to spin. Because the turbine is mounted on one end of a shaft that has a radial air compressor mounted on an opposite end, rotary action of the turbine also causes the air compressor to spin. The spinning action of the air compressor causes intake air to enter a compressor housing and to be pressurized, or boosted, before the intake air is mixed with fuel and combusted within an engine combustion chamber.
To reduce friction between and to extend the useful life of the turbocharger or other turbomachinery, foil bearings may be used to support rotating components of the turbine engines, turbochargers, and the like. Generally, a foil bearing includes a journal mounted to the rotating component and a cylindrical top foil disposed around the journal. The journal and top foil are configured such that when the rotating component rotates at an optimum operational speed, the foil and the journal separate from each other to form an air gap. As the air gap between the foil and the journal grows, pressurized air is drawn in to serve as a load support and act as a lubricant to the rotating component and surrounding static components.
In the absence of the pressurized air between the journal and the top foil, the two components will come into contact with each other. Thus, to protect the components from premature wear, one or more of the components may include a solid lubricant coating thereon. Some known solid lubricants include metal oxides, such as chromium oxide, nickel oxide, cobalt oxide, titanium oxides, or oxide blends, or mixtures thereof. Other solid lubricants may include metals that are bonded with metal oxides, which are then blended with a fluoride.
Although the aforementioned solid lubricants provide excellent lubrication between rubbing components and are chemically stable at elevated temperatures (e.g., temperatures above 480° C.), they may be improved. For example, because the aforementioned solid lubricants are typically plasma-sprayed onto the component, thin components (e.g., components having a thickness of less than about 0.01 cm, such as foils of a foil bearing system) may become distorted during the plasma spray process. Hence, application of the solid lubricants may be limited to thicker components. In the case of the foil bearing system, the thicker component may comprise a moving part of the foil bearing (e.g., the journal or thrust runner), and wear may occur on the journal or thrust runner, which may create an imbalance in the foil bearing system. For example, when the foil bearing system is pressurized during normal operation and subjected to high intermittent loads, the coating on the foil component can come in contact with the journal. However, because the moving part of the foil bearing may not be capable of being “broken in” (e.g., where minor distortions on a surface of the moving part of the foil bearing are abraded away), the moving part may continue to create imbalance in the foil bearing, which may decrease load capacity and stability of the foil bearing over time. For example, in some cases, a “high spot” may be present on a stationary part of the foil bearing (e.g., a mating foil), and a coating on the moving part of the foil bearing system (e.g., the journal) may wear over the complete circumference. As a result, the wear on the foil bearing system may become amplified, and the surface finish of the foil bearing system may undesirably increase thereby reducing the load capacity and/or decreasing lifetime.
In another example, plasma-sprayed solid lubricant coatings may have a relatively high porosity and large particle size, which may render the coatings difficult to polish to a desired surface finish which in turn may limit load capacity of the foil bearing. Furthermore, traditional solid lubricant coatings may be limited to use in temperature environments below about 300° C., and thus, extensive cooling may be required if the foil bearings are to be employed at temperatures above 300° C. However, even if the components are suitably cooled, coking deposits may accumulate during operation and may interfere with the operation and/or decrease the lifetime of the foil bearings.
Accordingly, it is desirable to have a solid lubricant that may be applied to thin components and may be polished to a surface finish of less than about 0.20 micron (8 microinches). Additionally, it is desirable for the solid lubricant and methods of applying the solid lubricant to be relatively simple and inexpensive to apply. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.